This invention relates in general to computers and associated displays. More specifically, the invention is directed to an interface for adapting a computer, designed to drive an NTSC-type monitor, to drive an RGB-type monitor.
Many computers, such as for example the Apple II E.TM. and the Apple II C.TM. (trademarks of Apple Computer, Inc., Cupertino, Calif.) provide composite or National Television System Committee (NTSC) video as their only video output for driving a monitor. This invention describes an arrangement whereby 100% compatibility is achieved when translating the composite video stream of the double density high resolution (HIRES) video mode of the Apple II.TM. series computers into a format suitable for driving an RGB-type monitor.
Computers until today have all interfaced to monitors which are designed for the NTSC standard. The reason for this has been economics. All television sets and television studio monitors must adhere to NTSC rules to guarantee compatibility among the transmitting stations and the many different brands of receivers on the market. The volume of NTSC type monitors produced on a daily basis has made them inexpensive for use as computer monitors. Their resolution (video fidelity), however, is unnecessarily limited by a set of air communication restrictions which really do not apply to computers.
Since computers today represent an increasing market force of their own, i.e., extremely high volumes, a new type of monitor, namely the RGB, has appeared in the marketplace at comparable pricing. RGB monitors are not constrained by air communication standards (since they are intended to be used with a single transmitter, i.e. the computer) and thus have much better resolution.
Composite video is regulated by a set of codes which were formulated for television transmission and reception by the National Television System Committee (NTSC). This standarization was required so that all television transmitters and receivers (televisions) would be compatible within the United States.
NTSC monitors are also known as "composite" video monitors, which stems from the regulations imposed by the NTSC. The regulations specify that the video stream must be composed of the superimposition of four separate signals merged into one. The four signals that make up the "composite" video signal are: (1) a composite synchronization signal, (2) a composite blanking signal, (3) a color burst signal, and (4) the actual video data.
The composite synchronization signal includes both vertical and horizontal synchronizations signals. This signal is needed by the television receiver to maintain picture stability with the transmitter as the video is scanned and "painted" on the screen.
The composite blanking signal includes both vertical and horizontal blanking signals. This signal is needed to blank the video gun while its in the retrace mode. A TV picture is painted on the screen line by line starting at the top left corner of the screen. The gun is turned on whenever it is appropriate to illuminate a portion of that line. Once the line has been finished, however, the gun must be positioned on the next line down. This repositioning (retrace) of the gun from the right hand side of the screen to the left hand side of the screen must be performed with the gun off. The blanking signal guarantees that the gun is off during the repositioning of the gun.
The video data is the visual information that is transmitted by the television station and which is to be displayed to the viewer. This video information modulates the video gun as it scans across the screen in a horizontal direction for each line of the picture. The gun either illuminates the screen or not depending on the video data transmitted. Once a horizontal line has been painted the gun is "blanked" and is forced to retrace to the next lower line. Once all lines for a particular frame have been scanned the gun must again be "blanked" as it retraces to the top leftmost part of the screen, before it may "paint" the next frame. Video data has two qualities: luminance and chrominance. Luminance (brightness) is directly proportional to the voltage level (magnitude) of the video signal. Chrominance (color) on the other hand is encoded using phase shift modulation techniques.
The color burst signal is transmitted during a small portion of each horizontal line while the gun is being "blanked". The color burst signal in the United States is standarized to 3.58 MHz. An internal oscillator in the television receiver locks to the exact phase of the color burst signal. The video data's phase shift differential to this internal oscillator is then obtained, and used to control the strength of the red, green and blue guns to generate a myriad of colors.
In RGB (red green blue) monitors three color guns are directly controlled, i.e. three separate signals must be supplied. Since direct control of the color guns is available, a color burst signal is not needed and all the color decoding circuitry found in composite monitors need not be present in RGB monitors. In the RGB system there is no need to encode and decode the color information, but rather it is controlled directly, a tremendous improvement in the video bandwidth is thus obtained.
For RGB monitors, the composite blanking signal is still needed and must be supplied to all three guns. The composite synchronization signal is also needed and, depending on the monitor, is either presented as a separate input or in composite form with one of the color gun inputs.
Two types of RGB monitors are presently available: analog and digital. Some monitor manufacturers include both options in one monitor. Analog monitors have only three inputs to control the three color guns. Since their input is analog, any gun may be controlled in a continuous fashion and thus an infinite number of colors may be displayed.
Digital monitors usually have four inputs to control the three color guns. These four inputs are digital and thus only sixteen possible colors may be obtained. The possible sixteen colors are "programmed" by the RGB monitor manufacturer. Some manufacturers today supply two different sets of sixteen colors selectable via an external switch. These two different sets of sixteen colors are targeted to support the color schemes of the Apple and IBM computers.
The Apple II.TM. series computers generate a video mode, known as double density high-resolution (HIRES), which may have as many as 560 different transitions during a single horizontal scan line of a frame. A complete screen (frame) consists of 192 such lines.
When the double density HIRES computer video mode is displayed by a monochrome (luminance only) NTSC monitor, the resolution is 560.times.192. This means that the brightness of 560.times.192 different locations (pixels) on the screen may be independently controlled. When such a video mode is displayed on a color (luminance and chrominance) NTSC monitor, however, the 560 transitions are interpreted (in sets of four) as color information by decoding by comparison with the color burst signal. The resolution is thus 140.times.192 with sixteen possible colors (only color-no luminance). As can be seen, therefore, the same video mode from the computer (double density HIRES) may be interpreted in two completely different ways by the monitor depending on the type of NTSC monitor used.